The rapid increase of smartphone use in recent years is spurring development of various application services and contents, and it is expected that this tendency is likely to be accelerated. In line with this tendency, various technologies for efficiently preparing for data overload caused by such various application services are being studied for use in cellular systems.
In order to meet the increasing demand for wireless data traffic since the commercialization of 4G communication systems, the development focus is on the 5th Generation (5G) or pre-5G communication system. For this reason, the 5G or pre-5G communication system is called a beyond 4G network communication system or post Long Term Evolution (LTE) system. Consideration is being given to implementing the 5G communication system in millimeter wave (mm Wave) frequency bands (e.g., 60 GHz bands) to accomplish higher data rates. In order to increase the propagation distance by mitigating propagation loss in the 5G communication system, discussions are underway about various techniques such as beamforming, massive MIMO, Full Dimensional MIMO (FD-MIMO), array antenna, analog beamforming, and large scale antenna. Also, in order to enhance network performance of the 5G communication system, developments are underway of various techniques such as evolved small cell, advanced small cell, cloud Radio Access Network (RAN), ultra-dense network, Device to Device (D2D) communication, wireless backhaul, moving network, cooperative communication, Coordinated Multi-Points (CoMP), and interference cancellation. Furthermore, the ongoing research includes the use of Hybrid FSK and QAM modulation and Sliding Window Superposition Coding (SWSC) as Advanced Coding Modulation (ACM), Filter Bank Multi Carrier (FBMC), Non-Orthogonal Multiple Access (NOMA), and Sparse Code Multiple Access (SCMA).
Meanwhile, the Internet is evolving from a human-centric communication network in which information is generated and consumed by humans to the Internet of Things (IoT) in which distributed things or components exchange and process information. The combination of the cloud server-based Big data processing technology and the IoT begets Internet of Everything technology. In order to secure the sensing technology, wired/wireless communication and network infrastructure, service interface technology, and security technology required for implementing the IoT, recent research has focused on sensor network. Machine to Machine (M2M), and Machine Type Communication (MTC) technologies. In the IoT environment, it is possible to provide an intelligent Internet Technology that is capable of collecting and analyzing data generated from connected things to create new values for human life. The IoT can be applied to various fields such as smart home, smart building, smart city, smart car or connected car, smart grid, health care, smart appliance, and smart medical service through legacy Information Technology (IT) and convergence of various industries.
Thus, there are various attempts to apply the IoT to the 5G communication system. For example, sensor network, Machine to Machine (M2M), and Machine Type Communication (MTC) technologies are implemented by means of 5G communication technologies such as beamforming, MEMO, and array antenna. The application of the aforementioned cloud RAN as a big data processing technology is an example of convergence between the 5G and IoT technologies.
Recently, Device to Device (D2D) communication (or direct communication) has been spotlighted for relieving increased base station load from large mobile contents by using the proximity of mobile communication terminals. D2D communication has been adopted as a study item of the current 3GPP LTE release 12, and RAN1 study started D2D standardization in January 2013. Meanwhile, RAN plenary agreed to end the D2D study item in February 2014 and start the D2D work item standardization from March 2014.
The LTE-based D2D communication technology may include D2D discovery and D2D communication.
D2D discovery is a process in which a UE checks the identities or interests of other UEs located nearby or advertises its identity or interest to other UEs located nearby. Here, the identity and interest may be represented by a UE identifier (ID), an application identifier, or a service identifier, and these can be configured diversely depending on the D2D service and operation scenario.
It is assumed that the protocol layers of the UE include a D2D application layer, a D2D management layer, and a D2D transport layer. The D2D application layer accommodates D2D service application programs running on the Operating System (OS) of the UE, the D2D management layer is responsible for the function of converting the discovery information generated by a D2D application program to a format suited to the transport layer, and the transport layer corresponds to the PHY/MAC layer of the LTE or Wi-Fi wireless communication standard. D2D discovery is performed with the following procedure. If the user executes the D2D application program, the application layer generates discovery information to the D2D management layer. The management layer converts the discovery information received from the application layer to a management layer message. The management layer message is transmitted through the transport layer of the UE, and the UEs receiving the message operate in the reverse order of the transmission process.
D2D communication is a communication method for exchanging traffic between UEs without passing through any infrastructure such as an eNB or Access Point (AP). D2D communication may be performed based on the result of the D2D discovery procedure (i.e., with the discovered UEs) or without D2D discovery procedure. Whether the D2D discovery procedure is required before D2D communication depends on the D2D service and operation scenario.
The D2D service scenarios may be categorized into commercial service or non-public safety service and public safety service. The services may include an innumerable number of examples such as advertisement, Social Network Service (SNS), game, and public safety service.
1. Advertisement: A communication network operator supporting D2D allows preregistered stores, cafes, movie theaters, and restaurants to advertise their identities to D2D users located within a short distance using D2D discovery or D2D communication. At this time, the interests may include advertisers' promotions, event information, and discount coupons. If the corresponding identity matches the interest of a user, the user may pay a visit to the corresponding store and collect more information through legacy cellular communication or D2D communication. In another example, a personal user may discover a taxi around him/her through D2D discovery and exchange data about a destination or fare through the legacy cellular communication or D2D communication.
2. Social Network Service (SNS): A user may send other users located within a short distance the user's application and interests in the corresponding application. At this time, the identity or interest used for D2D discovery may be a buddy list or the application identifier. The user may share contents such as photos and videos with the neighboring users through D2D communication after D2D discovery.
3. Game: The user discovers other users and game applications for playing a mobile game with neighboring users through the D2D discovery procedure and performs D2D communication for transmitting data necessary for the game.
4. Public Safety Service: Police and firefighters may use the D2D communication technology for public safety purposes. That is, in the case where cellular communication is not available because of a cellular network breakage caused by an emergency situation such as a fire or a landslide or a natural disaster such as an earthquake, volcanic eruption, or tsunami, the police and firefighters may discover neighboring colleagues or share the emergency situation information with neighboring users using the D2D communication technology.
The current 3GPP LTE D2D standardization is directed to both D2D discovery and D2D communication, but the standardization range is different. Commercial use is an aim of D2D discovery; thus, it should be designed to operate in the network coverage of an eNB. That is, D2D discovery is not supported in the situation where no eNB exists (or out of the coverage of an eNB). Public safety and disaster network services are aims of D2D communication rather than commercial use; thus, it should be supported in and out of network coverage and in partial network coverage of an eNB (communication in the situation where some UEs are located in the coverage of the eNB and other UEs are located out of the coverage of the eNB). Accordingly, public safety and disaster network services are provided through D2D communication without support of D2D discovery.
A characteristic of both D2D discovery and D2D communication is that they are performed in LTE uplink subframes. That is, the D2D transmitter transmits D2D discovery signals and data for D2D communication in the uplink subframes, and the D2D receiver receives them in the uplink subframes.
In the current LTE system, the UE receives data and control information from the eNB through downlink and transmits data and control information to the eNB through uplink, but the operation of the current D2D transmitter/receiver differs from those in the legacy LTE. For example, the D2D function-enabled UE has an orthogonal frequency division multiplexing (OFDM)-based receiver to receive the downlink data and control information from the eNB and a single carrier-frequency division multiplexing (SC-FDM)-based transmitter to transmit uplink data and control information to the eNB. However, since the D2D UE has to support both the cellular mode and D2D mode, it has to have an extra SC-FDM receiver to receive the D2D data and control information in uplink as well as the OFDM-based receiver and the SC-FDM-based transmitter.
The current LTE D2D defines two types of D2D discovery schemes according to resource allocation scheme.
1. Type 1 discovery: The eNB broadcasts the uplink resource pool available for D2D discovery in a System Information Block (SIB) for all D2D UEs within the cell under its control. At this time, the resource size available for D2D (e.g., x consecutive subframes) and period of resource (e.g., repeating at every y seconds) are informed. The sending D2D UEs that have received the information select the resource for transmitting D2D discovery signals in a distributed manner. Meanwhile, the receiving D2D UEs have to receive all D2D discovery signals transmitted in the resource pool and including SIB information.
2. Type 2 discovery: The eNB notifies the receiving D2D UEs of the discovery resource pool using the SIB. The discovery signal resources for the sending D2D UEs are scheduled by the eNB. At this time, the eNB may perform scheduling in a semi-persistent manner or a dynamic manner.
Like D2D discovery, D2D communication can be categorized into two types according to resource allocation scheme.
1. Mode 1: The eNB notifies the D2D transmitter of the data transmission resource for D2D communication directly.
2. Mode 2: The eNB notifies the D2D transmitter of the available resource pool, and the UEs select the resource in a distributive manner in the resource pool for transmission.
Meanwhile, in the LTE D2D standard, consideration has been given to using Frequency Division Multiplexing (FDM) for D2D terminals, and it has been agreed to apply frequency multiplexing on Physical Uplink Control Channel (PUCCH) as the uplink feedback channel of legacy cellular terminals and Physical Uplink Shared Channel (PUSCH) for D2D discovery and communication even in the subframe allocated for D2D communication purposes. It is assumed that maximum power transmission is used to increase coverage of D2D discovery and D2D communication in the LTE D2D standardization.
In the Type 1 discovery and Mode 2 communication, the eNB allocates D2D resources by means of a System Information Block (SIB) transmitted in a downlink subframe. For example, the eNB scrambles the D2D discovery signal and D2D communication (data transmission) resource allocation information with a System Information-Radio Network Temporary Identifier (SI-RNTI) or D2D-RNTI and transmits the scrambled signal through a Physical Downlink Control Channel (PDCCH). All of the UEs located within the cell know the SI-RNTI that is used by the UEs to acquire the allocation information included in the SIB transmitted through the PDCCH. The SIB is transmitted through a Physical Downlink Shared Channel (PDSCH). That is, the UE acquires resource allocation information of the SIB for PDSCH from the PDCCH and decodes to acquire resource allocation information for transmitting D2D discovery signals.
Once the resources for the D2D discovery signal transmission or the D2D data transmission are allocated, the UEs having D2D discovery signals or D2D data to transmit select radio resources (Resource Blocks (RBs)) for transmitting D2D discovery signals or D2D data in a distributed manner on the allocated resources. The distributed resource selection may be categorized into two schemes: random resource selection and energy sensing-based resource selection.
1. Random resource selection
(1) The UE selects resources randomly in the D2D resource pool allocated by the eNB for transmission.
2. Energy sensing-based resource selection
(1) The UE measures the energy level of all radio resources (Resource Blocks (RBs) during a predefined period. Here, the predefined period is a subset of the D2D resource pool.
(2) The UE selects an RB having the lowest energy level to transmit peer discovery information or data, or it selects randomly one of the RBs at which the energy level is equal to or less than a predetermined threshold value (i.e., lowest x %, e.g., 5%) to transmit the discovery information or data.